Regeneration of Etch Solutions Containing Trivalent Manganese in Acid Media

ABSTRACT

A method of regenerating an etch solution comprising a metastable complex of manganese(III) ions in a strong acid is described in which at least a portion of the manganese(III) ions in the metastable complex have been destabilized, causing them to disproportionate into manganese dioxide and manganese(II) ions. The method includes the steps of i) adding an effective amount of a reducing agent to the solution; ii) allowing the reducing agent to react with the solution to cause manganese dioxide to dissolve; and (iii) applying an electrical current to regenerate manganese(III) ions in the solution.

FIELD OF THE INVENTION

The present invention relates generally to improved processes forregenerating etch solutions containing trivalent manganese.

BACKGROUND OF THE INVENTION

It is well known to plate non-conductive substrates, (i.e. plastics)with metal for a variety of purposes. Plastic moldings are relativelyinexpensive to produce and metal plated plastic is used for manyapplications. For example, metal plated plastics are used for decorationand for the fabrication of electronic devices. An example of adecorative use includes automobile parts such as trim. Examples ofelectronic uses include printed circuits, wherein metal plated in aselective pattern comprises the conductors of the printed circuit board,and metal plated plastics used for EMI shielding. ABS resins are themost commonly plated plastics for decorative purposes while phenolic andepoxy resins are the most commonly plated plastics for the fabricationof printed circuit boards.

There are typically many stages involved in the plating of plastics. Thefirst stage involves etching the plastic in order to provide mechanicaladhesion of the subsequent metallic coatings and to provide a suitablesurface for the adsorption of the palladium colloid catalyst which isusually applied in order to catalyze deposition of the initial metalliclayer from an autocatalytic nickel or copper plating process. Followingthis, deposits of copper, nickel and chromium are typically applied.

The initial etching of the plastic components is an essential element ofthe overall process. However, only certain types of plastic componentsare suitable for plating. The most common. types of plastic forelectroplating are acrylonitrile/butadiene/styrene (ABS) or a blend ofthis material with polycarbonate (ABS/PC). ABS consists of two phases.The first is a relatively hard phase consisting of anacrylonitrile/styrene copolymer and a softer polybutadiene phase.

Currently, this material is etched almost exclusively using a mixture ofchromic and sulfuric acids, which is highly effective as an etchant forABS and ABS/PC. The polybutadiene phase of the plastic contains doublebonds in the polymer backbone, which are oxidized by the chromic acid,thus causing complete breakdown and dissolution of the polybutadienephase exposed at the surface of the plastic which gives an effectiveetch to the surface of the plastic.

One problem with the traditional chromic acid etching step is thatchromic acid is a recognized carcinogen and is increasingly regulated,requiring that wherever possible, the use of chromic acid is replacedwith safer alternatives. The use of a chromic acid etchant also haswell-known and serious drawbacks, including the toxicity of chromiumcompounds which makes their disposal difficult, chromic acid residuesremaining on the polymer surface that inhibit electroless deposition,and the difficulty of rinsing chromic acid residues from the polymersurface following treatment. Additionally, hot hexavalent chromic acidsolutions are naturally hazardous to workers. Burns and upperrespiratory bleeding are common in workers routinely involved with thesechrome etch solutions. For many years, alternative processes have beensought, but an effective commercially suitable process has not yet beendeveloped.

Early attempts to replace the use of chromic acid to etch plastictypically involved the use of permanganate ions as an alternative tochromic acid. The use of permanganate was described by U.S. Pat. No.4,610,895 to Tubergen et al., the subject matter of which is hereinincorporated by reference in its entirety. Later, the use ofpermanganate was described in combination with an ionic palladiumactivation stage as set forth in U.S. Pat. Pub. No. 2005/0199587 toBengston, the subject matter of which is herein incorporated byreference in its entirety. The use of permanganate solutions incombination with perhalo ions (such as perchlorate or periodate) wasdescribed, for example in U.S. Pat. Pub. No. 2009/0092757 to Satou, thesubject matter of which is herein incorporated by reference in itsentirety. Finally, International Publication No. WO2009/023628 toSchildman et al., the subject matter of which is herein incorporated byreference in its entirety, described the use of permanganate ions in theabsence of alkali metal or alkaline earth metal cations.

However, all of these attempts to etch plastic using permanganate ionshave not been capable of producing etch characteristics which matchthose obtained by the use of chromic acid and the stability of theseetch solutions is poor, resulting in the formation of manganese dioxidesludge.

As is readily seen, while many etching solutions have been suggested asa replacement for chromic acid in preparing non-conductive substratesfor metallization, none of these processes have proven satisfactory forvarious economic, performance and/or environmental reasons and none ofthese processes have achieved commercial success or been accepted by theindustry as suitable replacements for chromic acid etching. In addition,the stability of permanganate based etching solutions is also poor.

SUMMARY OF INVENTION

U.S. application Ser. No. 13/795,382 to Pearson et al., the subjectmatter of which is herein incorporated by reference in its entirety,describes an etch solution comprising trivalent is manganese ions in astrong acid that is capable of etching ABS. Pearson found that trivalentmanganese is unstable and is highly oxidizing, with a standard redoxpotential of 1.51 versus a normal hydrogen electrode. In solution, itvery rapidly disproportionates to manganese dioxide and divalentmanganese via the following reaction:

2 Mn³⁺+2 H₂O MnO₂ +Mn²⁺+4 H   (1)

However, Pearson found that in strong sulfuric acid solutions, trivalentmanganese ions become meta-stable and form a cherry purple/red coloredsulfate complex which provide a suitable medium for etching of ABS andwhich has many advantages over previously patented chromium-free etchingsolutions.

Unfortunately, because the solution is based on very strong sulfuricacid, the solution has a tendency to absorb water, which destabilizesthe manganese(III) ions, causing them to disproportionate into manganesedioxide and manganese(II) ions. Additionally, at higher concentrationsof Mn(III) ions, there is an increased tendency to create small amountsof manganese dioxide over time. Thus, this solution is also prone togenerating manganese dioxide sludge during operation over time.

Because trivalent manganese etch solutions based on strong acid havebeen shown to provide a suitable medium for etching platable plastics,it would be desirable to solve the problem of manganese dioxide sludgegeneration to extend the life of the etch solution and provide acommercially acceptable etch solution based on trivalent manganese.

It is an object of the invention to provide an etchant for plasticsubstrates that does not contain chromic acid.

It is another object of the present invention to provide an etchant forplastic substrates that is commercially acceptable.

It is another object of the present invention to provide an etchant forplastic substrates that is based on manganese ions.

It is still another object of the present invention to provide a meansfor regenerating manganese(III) ion-based etchants to extend their life.

To that end, in one embodiment, the present invention relates generallyto a method of regenerating an etch solution comprising a metastablecomplex of manganese(III) ions in a strong acid, wherein at least aportion of the manganese(III) ions have been destabilized, causing themto disproportionate into manganese dioxide and manganese(II) ions, themethod comprising the steps of:

-   -   a) Adding an effective amount of a reducing agent to the        solution;    -   b) Allowing the reducing agent to react with the solution to        cause manganese dioxide to dissolve; and    -   c) Applying an electrical current through an anode and a cathode        in the solution to regenerate manganese(III) ions in the        solution from the manganese(II) ions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As discussed above, solutions based on strong sulfuric acid have shown atendency to absorb water, which destabilizes the manganese(III) ions,causing them to disproportionate into manganese dioxide andmanganese(II) ions. In addition, there is an increased tendency tocreate small amounts of manganese dioxide over time. As a result, thesolutions are prone to generating manganese dioxide sludge duringoperation. Based thereon, the inventors of the present invention havediscovered that it is possible to regenerate solutions containingmanganese(III) ions in which manganese dioxide sludge has built up to anunacceptable level.

The present invention relates generally to a method of regeneratingsolutions containing manganese(III) ions and an inorganic acid that havebuilt up sludge (manganese dioxide) to an excessive level. The methodinvolves the addition of a reducing agent, optionally heating thesolution, and then applying an electrical current through an anode and acathode in the solution to regenerate manganese(III) ions.

As described in related application Ser. No. 13/795,382 to Pearson etal., the subject matter of which is herein incorporated by reference inits entirety, trivalent manganese can readily be produced byelectrolysis at low current density of divalent manganese ions in astrong acid solution, preferably a strong sulfuric acid solution, mostpreferably an at least 8M sulfuric acid solution. Further, a solution oftrivalent manganese ions in a strongly acidic solution is capable ofetching ABS and is thus a suitable replacement for chromic acid etchingsolutions.

As described above, trivalent manganese is unstable and is highlyoxidizing (standard redox potential of 1.51 versus normal hydrogenelectrode). In solution, it disproportionates to manganese dioxide anddivalent manganese. However, in a strong sulfuric acid solution, thetrivalent manganese ion becomes meta-stable and forms a cherrypurple/red colored sulfate complex. This sulfate complex is a suitablemedium for the etching of ABS and has many advantages over chromium-freeetches of the prior art. In addition, while both phosphoric acid andsulfuric acid have been shown to be suitable for use in the electrolyte,in a preferred embodiment, the acid is sulfuric acid.

The stability of manganese(III) ions in strong sulfuric acid providesthe following advantages in use:

-   -   1) Because the Mn(III) ions are fowled from Mn(II) at a low        current density, the power requirements for the process are        typically very low.    -   2) Because the anode operates at a very low current density, a        small cathode in relationship to the anode area can be used to        prevent cathodic reduction of the Mn(III) ions. This obviates        the need for a divided cell and makes the engineering of an        etchant regeneration cell simpler.    -   3) Because the process does not produce permanganate ions, there        is no possibility of producing manganese heptoxide in the        solution (this is a considerable safety hazard as it is        violently explosive).    -   4) Because of the high stability of the Mn(III) ions in strong        sulfuric acid, the etchant can be sold ready for use. The        etchant requires only a small regeneration cell at the side of        the tank in order to maintain the Mn(III) content of the etch        and prevent the build-up of Mn(II) ions.    -   5) The electrolytic production of Mn(III) in accordance with the        present invention does not produce any toxic gases. While some        hydrogen may be produced at the cathode, owing to the low        current requirements, this would be less than that produced by        many plating processes.

The concentration of sulfuric acid in the Mn(III) etchant is preferablyat least 8 molar, more preferably between about 9 and about 15 molar.The concentration of sulfuric acid is important in the process and inorder to obtain rapid rates of etching on ABS plastic, it is necessaryto use a high concentration of acid. Below a concentration of about 8molar, the rate of etch becomes slow and above about 14 molar, thesolubility of manganese ions in the solution becomes low. Additionally,very high concentrations of sulfuric acid tend to absorb moisture fromthe air and are hazardous to handle. Thus, the concentration of sulfuricacid is most preferably between about 12 and 13 molar, which is diluteenough to allow the safe addition of water to the etch and strong enoughto optimize the etch rate of the plastic. At this concentration ofsulfuric acid, up to around 0.08M of manganese sulfate can be dissolvedat the preferred operating temperature of the etch. For optimal etching,the concentration of manganese(III) ions in solution should be as highas it is feasible to achieve.

The manganese(II) ions are preferably selected from the group consistingof manganese sulfate, manganese carbonate and manganese hydroxidealthough other similar sources of manganese(II) ions known in the artwould also be usable in the practice of the invention. The concentrationof manganese(II) ions may be in the range of between about 0.005 molarup to saturation. The electrolyte may also comprise colloidal manganesedioxide. This may form to some extent as a natural result ofdisproportionation of manganese(III) in solution, or may be addeddeliberately. Manganese(III) ions can be conveniently generated byelectrochemical means by the oxidation of manganese(II) ions. Inaddition, it is generally preferable that the electrolyte not containany permanganate ions.

As further described in U.S. Ser. No. 13/795,382, it is also possible toincrease the amount of manganese that can be dissolved in the bath byreplacing a portion of the sulfuric acid with another acid in which themanganese ions may be more soluble. Acids Which would have both thenecessary stability to oxidation and the ability to increase thesolubility of manganese ions in the bath are methane sulfonic acid andmethane disulfonic acid.

The electrochemical oxidation of manganese(II) ions to manganese(III)ions comprises the steps of:

-   -   providing an electrolyte comprising a solution of manganese(II)        ions in at least one acid in an electrolytic cell wherein the        electrolytic cell comprises an anode and a cathode;    -   applying a current between the anode and the cathode; and    -   oxidizing the electrolyte to form manganese(III) ions, wherein        the manganese(III) ions form a metastable complex.

Once the electrolyte has been oxidized to form the metastable complex,the platable plastic may be immersed in the metastable complex for aperiod of time to etch the surface of the platable plastic. In oneembodiment, the platable plastic is immersed in the metastable complex atemperature of between 30 and 80° C. The rate of etching increases withtemperature and is slow below 50° C.

The upper limit of temperature is determined by the nature of theplastic being etched. ABS begins to distort above 70° C., thus in apreferred embodiment the temperature of the electrolyte is maintainedbetween about 50 and about 70° C., especially when etching ABSmaterials. The time period of the immersion of the plastic in theelectrolyte is preferably between about 10 to about 30 minutes.

The present invention relates generally to a method of regenerating anetch solution comprising a metastable complex of manganese(III) ions ina strong acid, wherein at least a portion of the manganese(III) ionshave been destabilized, causing them to disproportionate into manganesedioxide and manganese(II) ions, the method comprising the steps of

-   -   a) Adding an effective amount of a reducing agent for the Mn(IV)        of the manganese dioxide to the solution;    -   b) Allowing the reducing agent o react with the solution to        cause manganese dioxide to dissolve; and    -   c) Applying an electrical current through an anode and a cathode        in the solution to regenerate manganese(III) ions in the        solution.

The inventors of the present invention have determined that in thestrongly acidic media, the use of hydrogen peroxide as a reducing agentcauses the manganese dioxide sludge to dissolve and reduces themanganese(IV) species to soluble manganese(II) ions according to thereaction:

MnO₂+H₂O₂+2 H⁺→Mn²⁺+O₂+2 H₂O   (4)

The reaction of hydrogen peroxide in this way is surprising because itis most commonly used as a strong oxidizing agent and thus it would notbe obvious to use hydrogen peroxide as a reducing agent in the mannersuggested herein. The manganese(III) ions can be reduced tomanganese(II) ions by the use of hydrogen peroxide in a strong acidalso. The lack of soluble oxidation products which may compromise thecontinued performance of the etching solution is also desirable andhydrogen peroxide is ideal in this respect, producing only water andgaseous oxygen. Other suitable reducing agents include oxalic acid,ascorbic acid, formic acid and other organic acids which will leave nosoluble by-products, producing only carbon dioxide and water.

In the case of hydrogen peroxide (35% w/w), the effective amount ofreducing agent added to the etchant solution to cause manganese dioxidesludge in the solution to dissolve is about 0.5 ml/L to about 10 ml/L,more preferably about 2 ml/L to about 7 ml/L of the reducing agent perliter of etching solution. In the case of oxalic acid or formic acid,the effective amount of reducing agent added to the etchant solution isin the range of about 1 to about 10 g/L, more preferably about 2 toabout 7 g/L.

In addition, the reducing agent may be added in stages. In other words,a portion of the reducing agent may be added and the solution allowed toreact before an additional quantity of the reducing agent is added tothe etch solution.

Optionally, but preferably, heat is applied to aid the reducing agent inreacting with the solution. Based thereon, the solution may be heated toa temperature of between about 30° C. and about 100° C., more preferablybetween about 60° C. and about 80° C.

Finally, it is noted that while the method described herein can be usedto treat an entire working etch solution bath in an electrolytic cell,the etch solution in the electrolytic cell may be spilt into portionsand each portion treated separately. For example, 10% of the bath may bediverted from the electrolytic cell and treated before being returned tothe electrolytic cell, which would enable continuous treatment of thebath in a “feed and bleed” type system.

The invention will now be illustrated with reference to the followingnon-limiting examples:

EXAMPLE 1

An etching solution was prepared according to the procedure set forth inrelated application Ser. No. 13/795,382, containing 0.08M manganesesulfate and 12.5M sulfuric acid. The solution was electrolyzed at 70° C.at low anodic current density using a platinized titanium anode and alead cathode. The anode area was 10 times the cathode area.

A cherry colored complex began to form and after several hours ofelectrolysis, the solution was a deep cherry purple color indicating theformation of the manganese(III) complex.

An ABS panel was immersed in the etch solution for 30 minutes, and thenrinsed and dried. The panel was examined under a scanning electronmicroscope and exhibited the topography expected of a well etched ABSpanel.

The solution was operated for more than 1 week at 70° C. and after thistime, a noticeable amount of brown sludge had formed in the solution.

The solution was then cooled to 30° C. and treated as follows:

-   -   1) 1 ml/L of 35% w/w hydrogen peroxide was added to the        solution. Gas quite vigorously evolved.    -   2) The solution was allowed to react for a further 30 minutes.    -   3) A further 2 ml/L of hydrogen peroxide was added and the        solution allowed to react further for another 30 minutes. After        this time, the appearance of the solution was turbid amber.    -   4) Heat was applied and the solution was heated to 80° C. for        about an hour. During this time, the solution became clear and a        pale amber color before turning a pale pink color.    -   5) A few extra drops of hydrogen peroxide were added and the        solution returned to the amber color before eventually fading to        a very pale pink, almost colorless, color characteristic of        divalent manganese ions.    -   6) The solution was then electrolyzed again in the electrolysis        cell under the same conditions as when the solution was first        prepared.    -   7) A cherry colored complex began to form and continued        electrolysis resulted in a very dark cherry purple color similar        to the original etch solution.

An ABS panel was immersed in the regenerated etch solution for 30minutes at 70° C. After treatment, the panel was examined by scanningelectron microscopy and the panel surface was observed to exhibit thetopography typical of that produced by a new etch solution.

EXAMPLE 2

An etching solution was prepared according to the procedure set forth inrelated application Ser. No. 13/795,382, containing 0.08M manganesesulfate and 12.5M sulfuric acid. The solution was electrolyzed at 70° C.at low anodic current density using a platinized titanium anode and alead cathode. The anode area was 10 times the cathode area.

A cherry colored complex began to form and after several hours ofelectrolysis, the solution was a deep cherry purple color indicating theformation of the manganese(III) complex.

An ABS panel was immersed in the etch solution for 30 minutes, thenrinsed and dried. The panel was examined under a scanning electronicmicroscope and exhibited the topography expected of an etched ABS panel.

The solution was operated for over a week at 70° C. and after this time,a noticeable amount of brown sludge had formed.

The solution was cooled to 30° C. and treated as follows:

-   -   1) 2 g/L of oxalic acid dihydrate was added to the solution.    -   2) The solution was allowed to react for 30 minutes.    -   3) A further 3 g/L of oxalic acid dihydrate was added and heat        was applied. The solution was heated to 80° C. for about an        hour. During this time, the solution became clear and eventually        turned a pale pink almost colorless color.    -   4) A few extra drops of hydrogen peroxide were added to remove        any excess oxalic acid. The solution turned an amber color        before eventually fading to a very pale almost colorless pink        color characteristic of divalent manganese ions.    -   5) The solution was then electrolyzed again in the electrolysis        cell under the same conditions as when the solution was first        prepared.    -   6) A cherry colored complex began to form and continued        electrolysis resulted in a very dark cherry purple color similar        to the original etch solution.

An ABS panel was immersed in the regenerated etch solution for 30minutes at 70° C. After treatment, the panel was examined by scanningelectron microscopy. The panel surface exhibited the topography typicalof that produced by the new etch solution.

EXAMPLE 3

A similar treatment was carried out using formic acid as the reducingagent and produced similar results to Examples 1 and 2.

What is claimed is:
 1. A method of regenerating an etch solutioncomprising a metastable complex of manganese(III) ions in a strong acid,wherein at least a portion of the manganese(III) ions have beendestabilized, causing them to disproportionate into manganese dioxideand manganese(II) ions, the method comprising the steps of: a. adding aneffective amount of a reducing agent for the Mn(IV) of the manganesedioxide to the etch solution; b. allowing the reducing agent to reactwith the etch solution to cause the Mn(IV) in the manganese dioxide tobe reduced to Mn(II) and to dissolve; and c. applying an electricalcurrent through an anode and a cathode in the etch solution toregenerate manganese(III) ions in the etch solution from manganese(II)ions.
 2. The method according to claim 1, wherein the reducing agent isselected from the group consisting of hydrogen peroxide, oxalic acid,formic acid and combinations of one or more of the foregoing.
 3. Themethod according to claim 2, wherein the reducing agent compriseshydrogen peroxide.
 4. The method according to claim 3, wherein theamount of hydrogen peroxide added to the solution is in the range ofabout 0.5 ml of hydrogen peroxide (35% by weight) per liter of etchsolution, to about 10 ml of hydrogen peroxide (35% by weight) per literof etch solution.
 5. The method according to claim 4, wherein the amountof hydrogen peroxide added to the solution is in the range of about 2 mlof hydrogen peroxide (35% by weight) per liter of etch solution, toabout 7 ml of hydrogen peroxide (35% by weight) per liter of etchsolution.
 6. The method according to claim 2, wherein the reducing agentcomprises oxalic acid or formic acid.
 7. The method according to claim6, wherein the amount of oxalic acid or formic acid added to thesolution is in the range of about 1 g/L to about 10 g/L.
 8. The methodaccording to claim 7, wherein the amount of oxalic acid or formic acidadded to the solution is in the range of about 2 g/L to about 7 g/L. 9.The method according to claim 1, wherein a portion of the amount ofreducing agent is added to the etch solution and the reducing agentallowed to react with the etch solution and, thereafter, an additionalamount of the reducing agent is added to the etch solution and allowedto react with the etch solution.
 10. The method according to claim 1,wherein the reducing agent is allowed to react with the etch solutionfor at least 30 minutes.
 11. The method according to claim 1, whereinthe reducing agent is allowed to react with the etch solution until allof the manganese dioxide in the solution has been dissolved.
 12. Themethod according to claim 1, comprising the step of heating the etchsolution after the reducing agent has been added to the etch solution.13. The method according to claim 12, wherein the etch solution isheated to a temperature of between about 30° C. and about 100° C. 14.The method according to claim 13, wherein the etch solution is heated toa temperature of between about 60° C. and about 80° C.
 15. The methodaccording to claim 1, wherein a portion of the etch solution is divertedfrom a process tank containing the etch solution into a separateelectrolytic cell and the portion of the etch solution which is divertedis regenerated and then recycled back into the process tank.
 16. Themethod according to claim 15, wherein the portion of the etch solutionthat is diverted from the process tank is approximately 10% of theworking volume of the process tank.
 17. The method according to claim15, wherein additional portions of etch solution are diverted from theprocess, whereby the etch solution can be continuously treated.